The invention relates to a single-phase synchronous motor comprising a two-pole permanent-magnet rotor of a magnetic material having a remanence B.sub.r, a specific density .rho., a rotor diameter d, a resulting detent torque of an amplitude M.sub.k1, and a mass moment of inertia J.
A single-phase synchronous motor of such a mechanical construction is known from the magazine "Feinwerktechnik und Messtechnik", 87 (1979/4, pages 163 to 169). Such motors are used for driving small domestic appliances such as citrus presses, can openers etc.
The problem of self-starting posed by these motors involves various aspects. Hereinafter, starting is to be understood to mean the transitional process which begins with the stationary condition of the motor and which ends with a condition characterized by a constant average angular velocity. This end condition may be a steady condition with periodic fluctuations of the instantaneous angular velocity, but there may also be low-frequency fluctuations which are superimposed on these periodic fluctuations. If, after application of the supply voltage, none of these conditions is reached, a motor will fail to start; that is, it will not be set into motion at all or its direction of rotation will change irregularly.
The starting voltage is defined as the lowest voltage of a voltage range above which the motor always starts without regard to the instant within the supply voltage cycle at which power is applied (hereinafter, the "switching-on instant"). This voltage range should be at least as wide as the required operating-voltage range. The load of the motor strongly influences the starting behavior, the starting voltage, and the width of the range.
Further, the starting behavior has static and dynamic aspects. With respect to the static aspects care must be taken, if necessary by the use of additional mechanical or other auxiliary circuits or devices, that the rotor cannot stall under the influence of friction torques or other loading torques in a position in which the instantaneous torque due to coil current, hereinafter referred to as the "current torque", which is a sinusoidal function of the rotor position, is zero. In the known motors this is generally achieved by an asymmetric shape of the stator-pole arcs. As a result of this asymmetry the magnetic reluctance torque of the rotor, also referred to as the detent torque, will not be zero in the position in which the current torque is zero. If the friction is not too high, this detent torque can rotate the rotor out of this positon, so that the current torque can provide the acceleration. In known models the angle of asymmetry is approximately 15.degree. but it may also reach values of approximately 30.degree..
With respect to the dynamic aspects it was assumed previously (ETZ A 87, March 1966, pages 171-175) that starting proceeded as a torsional vibration of increasing amplitude. Starting was then assumed to take place above an amplitude value of 180.degree..
Subsequently, the opinion was held that the detent torque impairs the starting process and it would be preferred, for example by dividing the rotor into two or more parts, to reduce or even completely eliminate this torque (DE-PS No. 14 88 267 corresponding to U.S. Pat. No. 3,433,987, "Der Elektromeister", Heft 1, 1965). Later it was found that "a specific, not too small, detent torque is necessary because otherwise the coil field has to be made too large to meet other requirements" (ETZ A 87, March 1966).
Investigations on synchronous motors, which are now manufactured in large production runs for use in small domestic appliances, have shown that starting actually proceeds as a jump from the stationary condition to the synchronous speed. Generally, this jump takes a time of approximately 6 msecs. This imposes requirements on the motor dimensioning which manifest themselves in the starting-time constant. Suitably, this starting-time constant lies in the range of approximately 2-6 msecs. The following requirement is valid: ##EQU3## Here: .omega..sub.e is the angular supply voltage frequency
J is the mass moment of inertia of motor and load PA1 U is the supply voltage PA1 E is the induced voltage PA1 Z is the coil impedance.
The detent torque is not involved in the concept of the starting-time constant. In deriving this starting requirement it has been assumed that after the voltage has been switched on, with a more or less irregular motion the rotor can reach a position in which the rotor magnetization extends substantially perpendicularly to the coil field and from which position this jump can be made.
This has always been the case with the known motors. If the starting-time constant becomes too small, for example if the voltage is too high, the rotor will perform very irregular motions accompanied by reversal of the direction. If the starting-time constant is too large, for example if the mass moment of inertia is too large, the rotor will not be set into motion and if mass moment of inertia is very large the rotor will merely vibrate.
If in known motors the voltage is increased starting from zero, the motor will initially perform vibrations of small amplitude. This amplitude then increases but generally does not exceed 20.degree. to 30.degree..
When the voltage is increased further this vibration rhythm of the known motors generally changes into a more or less regular rotation with or without direction reversal. The voltage at which this happens can be calculated to a close approximation, using the value of the starting-time constant. Hereinafter, this voltage is referred to as the breakaway voltage. Generally, the breakaway voltage increases when the inertial mass is increased, for example, by means of a load.
The starting voltage as defined above may be higher than the breakaway voltage depending on the stability requirements imposed on the rotor motion. Although instantaneously the synchronous angular velocity is exceeded, the motor cannot sustain a rotation in a specific direction and with constant average angular velocity. All single-phase synchronous motors known to date exhibit such a behavior.
For the miniaturization of these motors and appliances, it is obvious to use high-energy magnet materials. When RES-Magnets (Rare Earth Sintered magnets) are used, instead of barium-ferrite or strontium-ferrite magnets, the remanance may be increased for example from approximately 350 mT (3500 G) to over 790 mT. The specific density then increases from approximately 4.8 to 8.2 g/cm.sup.3. This step has a favorable effect on the starting-time constant, resulting in a reduction of this constant when the rotor dimensions remain the same. Reducing the rotor diameter has a similar effect. It also results in a reduced starting-time constant.
Nevertheless it is found with motors using high-energy magnet materials that, for specific initial values of the applied supply voltage, related to specific switching-on instants, the breakaway voltage is substantially higher than expected in view of the above considerations with regard to the starting-time constant.
There are several instantaneous voltage values at the switching-on instant which lead to a normal starting process as described above. However, at other switching-on instants the rotor will be locked in a condition in which it oscillates about the stationary position; these oscillations are referred to hereinafter as boundary oscillations. Only when the voltage is substantially higher than anticipated based on the starting-time constant, can the rotor break away from this condition of oscillation.
The occurrence of these boundary oscillations far above the voltage determined by the starting-time constant is new for the single-phase synchronous motors described here.